Flow meters known in the prior art include those of the "piston-spring load" type are disclosed in U.S. Pat. Nos. 3,218,853; 3,805,611; 3,979,955 and 4,389,901. Other flow meters utilize a piston-magnet arrangement, for example U.S. Pat. No. 2,638,582, or a spring-loaded plate construction, for example U.S. Pat. No. 3,554,031. Still others disclose the use of a float structure disposed within a conically shaped conduit, for example, U.S. Pat. No. 3,759,099. Finally, U.S. Pat. No. 4,122,757 discloses a fluid flow meter utilizing a venturi-shaped throat and spring-loaded sliding member movable inside a cylinder in response to fluid flow pressure.
With one exception, the foregoing prior art devices all suffer from a primary disadvantage in that they measure only fluid flow rate, while permanently installed in a pipeline, i.e., they cannot be easily converted to use or used to measure vacuum air flow rate as well in a portable configuration. In contrast, the present invention provides a highly sensitive fluid flow meter which allows direct reading of both air flow rate and vacuum. In addition, this meter is portable and can be read in any position horizontal or vertical. Although U.S. Pat. No. 3,979,955 discloses a fluid flow meter which can be used in any flow direction, it cannot be used to measure both air flow and vacuum.
There are other objects and advantages of the present invention, and those will become apparent upon reading the following description in light of the accompanying drawing.